A thermoplastic polyester series resin (hereinafter referred to as PET) is known as an engineering plastic and has excellent properties which are lacking in polystyrene and polyethylene. For example, PET has a large rigidity, a good foam stability, and an excellent heat resistance of enduring 200.degree. C. Thus, it was tried to make tough heat-insulating materials, cushioning materials, package containers, food containers, etc., having good heat resistance by foaming PET. However, different from polystyrene and polyethylene, it was not easy to foam PET. Even when PET can be foamed, the foamed material has a low expansion ratio only.
The reason that PET is reluctant to be foamed is the crystallinity of PET and is that it is difficult to keep the viscosity of PET suitable for foaming when PET is melted. More specifically, when PET is heated, PET is suddenly softened at a certain temperature to become a liquid having a low viscosity. That is, for PET, the temperatures suitable for foaming are limited to a very narrow temperature range and, hence, it is difficult to keep PET at a temperature suitable for foaming. Also, when PET becomes a liquid having a low viscosity, a gas acting as a foaming agent is immediately released therefrom. Therefore, even when PET can be foamed, the expansion ratio is very low and it is difficult to make foamed materials of PET of low density foamed at a high expansion ratio.
For solving these problems, various attempts have been made. For example, it was proposed to mix an epoxy compound with PET or mix a metal belonging to group Ia or group IIa of the periodic table with PET. However, by these proposals, large-sized foamed materials foamed at a high expansion ratio could not be obtained.
Also, JP-A-59-135237 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and U.S. Pat. No. 4,462,947 propose the use of high-molecular weight chain aromatic polycarbonate as a foaming agent for obtaining PET materials. However, by these techniques, foamed materials having a high expansion ratio cannot be obtained. In fact, in the above-cited patents, as an example of the foamed material obtained, only a material having a density of about 0.83 g/cm.sup.3 is described.
As other attempt, JP-A-55-2045 describes a method of obtaining a foamed material having a low density by introducing high-temperature PET containing a foaming agent directly after being extruded from an extruding machine into a reduced pressure zone and foaming it under reduced pressure. However, since high-temperature PET containing a foaming agent generally has uneven surfaces by foaming and has an irregular external form, it is not easy to construct a reduced pressure zone capable of continuously passing such high-temperature PET. In particular, it is difficult to make sufficiently reduced-pressure sealing of the outlet of the foamed material. Accordingly, the aforesaid method has a disadvantage of being not easily practiced.